1. Field of the Invention
The present invention relates to a document reading apparatus capable of reading a document which moves along a predetermined conveyance path and a related data transfer method.
2. Description of the Related Art
Conventionally, a document reading apparatus can be installed on or incorporated into a digital copying machine, a facsimile machine, or a scanner device. A document reading apparatus installed on a copying machine which operates in a “document feeding-reading” mode is known. According to this operation mode, the sheets of a document stacked on a document tray are conveyed one after another to a document positioning glass plate, each sheet is exposed by an exposure apparatus fixed on a conveyance path, and an image of the exposed sheet is read by an image sensor.
In recent years, to improve the reading efficiency of the document reading apparatus that performs the above-described document feeding-reading operation, it has been effective to reduce a distance between two consecutive sheets (referred to as “sheet-to-sheet interval”) when the sheets stacked on the document tray are conveyed one after another along a conveyance path. This technique can increase the number of readable sheets per unit time.
For example, as discussed in Japanese Patent Application Laid-Open No. 2005-324872, the sheet-to-sheet interval can be reduced by quickly starting a feeding operation of a sheet upon detecting the trailing edge of a preceding sheet. The conveying speed of a document can be reduced without deteriorating the reading efficiency.
However, according to the above-described conventional document reading apparatus, the following problems occur if a plurality of sheets is continuously read and read image data are transferred in real-time to a printer connected to the document reading apparatus.
Namely, to keep a certain sheet interval between a preceding sheet and a following sheet, the time available for transferring the image data becomes shorter. Thus, the image data needs to be transferred at a clock frequency higher than a pre-designated clock frequency required to attain an intended reading efficiency.
If the entire system operates at a higher clock frequency, various problems (e.g., temperature rise, increase in electric power consumption, and generation of radiation noises) occur. Realizing a timing design for an application specific integrated circuit (ASIC) or other IC that performs image processing also becomes difficult.